T41EP::QSD module::Sallen-Key filters

Author: Wald 3Z6AEF [08 Sep 2022]

Sampling frequency

The PCM1808 ADC has guaranted max. 96 kHz sampling frequency (according to TI datasheet). In T41EP project (as in DD4WH Convolution RX) the chip is „overclocked” to 192kHz (Frank experimented with freq up to 256kHz and it worked, supposedly...). I don't like overclocking, but... if it works (even if not always) then why not 😉 It's just experimenting!

Antialiasing

But I'm a bit confusing with all that issues about aliasing and anti-alias filters... How important is it?
Reading theBook ^[1] (footnotes at the end of page), I states that this question IS quite important - a lot of information about this I found on many pages: 49, 60-61, 132, 149-151.

Anti-alias filter

On page 60 we read:

Our anti-alias filters are Sallen-Key third order active filter[...] Figure 3-20 shows a typical frequency response plot of our anti-alias filter[...]”.

On that figure we see Bode plot of Sallen-Key 2^nd order LP filter with 3 dB cut-off at about 380 kHz.

But on page 132 we read:

The final step, after the QSD, as shown in the circuit diagram in Figure 8-7, is to pass the signal to a low pass filter to remove high frequency components, that are above 200kHz[...]The filter is accopmlished by means a of pair of Sallen-Key analog Op Amp (i.e. U2) filters, configured as 3^rd order LP filters[...]

OK, lets check the diagram... Yes, it is 2nd order Sallen-Key LP. Where is their cut-off frequency? Using my favorite japanese calculator ^[2], for given filter's elements values:

Sallen-Key_elements

I got this:

Sallen-Key_results

Sallen-Key_Bodeplot

As we see, the cut-off frequency is about 460kHz (yes, I measured it... ^[3]), but not 200kHz as theBook states.

PCM1808 module

PCM1808module(small)

And last but not least...

PCM1808 chip has built in analog filters (in both channels), but their 3dB antialiasing cut-off frequency is declared as 1.3 MHz - not so useful... In datasheet (Fig.26, note 5)^[4] there is an advice:

Optional external antialiasing filter could be required, depending on the application[...].

Yeah... In fact, we have such a filter on chineese PCM1808 module. It is simple passive RC 1^st order low pass filter with R=10kOhm and C=330pF (measured... quite tiny elements 0603! ^[5]) - so 3dB cut-off frequency is about... 48,22 kHz (yes! once again I've using my favorite japaneese filter calculator ^[2:1] 😉

Experimenter's doubts...

My questions for you, smarter guys, are:

What 3 dB cut-off frequency should be used in anti-alising filter in T41EP RX path before ADC sampling?
Should I remove or change passive RC low pass elements on PCM1808 module? Is it worthwhile or needed?
Whether the elements in Sallen-Key filter on T41EP QSD module need to be changed?

Links and footnotes below:

theBook : Albert F. Peter AC8GY, Dr. Jack Purdum W8TEE, Theory and construction of the T41-EP Amateur Radio SDT, print 24224, Middletown, DE, 03 May 2022 ↩︎
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm ↩︎ ↩︎
3Z6AEF testing of T41 QSD Sallen-Key filter:
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PCM1808 datasheet : https://www.ti.com/lit/gpn/pcm1808 ↩︎
3Z6AEF measurement of PCM1808 module LP filter capacitor

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